i have the same problem:
around 3000 files in one folder (images around 100 - 500 kb)
displaying the folder in web takes me around one minute.
this issue on GitHub was closed but I think it should be reopened as this performance is still an issue. I have a folder with over 41,000 pictures in it and it is not practical for me to split it up into folders of a few hundred pictures each just for the app to work properly. iCloud is able to handle the same load without crashing in the web interface.